Its not about patents. Believe what you want but there is a reason nobody else is doing x86 or ARM chips unless they are allowed by the owner.
I know for example that Berkley when thinking pre-RISC-V that they had a deal with Intel about using x86-64 for research. But they were not able to share the designs.
Are you sure, especially considering China?
I doubt there is any legal barrier, because there are a few existing projects with x86 cores on an FPGA, as well as some SoCs. Here's a 486: https://opencores.org/projects/ao486
As for opencores, yes you can design them, but do any companies making commercial products sell them?